1. Field of the Invention
The present invention relates to a tetravinylpyrazine compound having a vinyl bond at the 2-, 3-, 5- and 6-positions of the pyrazine ring respectively, a method for preparing the pyrazine compound and an electroluminescent element and nonlinear optical material obtained using the pyrazine compound.
2. Description of the Prior Art
Due to the recent marked progress in the information industry and information-oriented society, there has been increasing requirements for members capable of displaying information which exhibit higher quality. Among elements for such members, electroluminescent elements (EL elements) which make use of an electroluminescence (EL) phenomenon have become of major interest lately because of their excellent properties such as good visibility.
There have been developed and already put on the market so-called intrinsic type EL elements such .as those in which inorganic fine particles are dispersed in a matrix of an organic substance and those obtained by sandwiching inorganic thin films of, for instance, znS between insulating thin films. However, these elements require high driving voltages and it is difficult to obtain elements which emit lights having wavelengths falling within, for instance, the blue range.
There have been known injection-type EL elements in addition to the intrinsic-type ones. These injection-type elements emit light through recombination of electrons and positive holes which have been injected into p-n junctions of, for instance, semiconductors. These elements are, for instance, characterized in that they can be operated at a low DC voltage and that they have high efficiencies of converting electric energy into light. Inorganic crystalline semiconductors such as GaP have principally been used to produce these injection-type EL elements. However, they suffer from the problems that they are limited in the colors of emitted lights and that it is difficult to enlarge the area of the elements.
For this reason, there have been recent been requirements for the development of techniques for producing EL elements which operate at a low driving voltage, can emit lights of any desired colors and have a large surface area.
Recently, there have been proposed novel injection-type EL elements which make use of thin films of organic compounds (see C. T. Tang, Appl. Phys. Lett., 1987, 51(12), p. 193) and have attracted much attention. This is because the colors of emitted lights can arbitrarily be selected due to the use of organic substances, the elements can be operated at a low DC voltage and an element having a large area can be obtained by a thin film-forming method such as a deposition or coating method. However, there still remains some problems. For example, the EL elements comprising the thin films of these organic compounds suffer from a problem of reduction in the brightness of the emitted lights when they are operated over an extended time, i.e., the problem of so-called deterioration.
One of the sources of such deterioration may be the degeneration of the organic compounds due to generation of heat. This is because, the efficiency of converting an electric energy into light is presently on the order of several percentages and most of the energy is converted into heat. Therefore, it is necessary to develop an element which makes use of an organic compound having good heat resistance and light-emitting efficiency in order to eliminate the problem of deterioration.
Incidentally, there have been proposed, as materials for organic thin layer EL elements, 2,5-distyryl pyrazine derivatives (see M. Nohara, Chem. Lett., 1990, p. 189), but the EL elements obtained from these materials suffer from the problem of low stability.
Further, non-linear optical devices which make use of tertiary non-linear optical effects such as optical bistable elements and optical gate switches have been anticipated as key devices for future ultrahigh speed electronic data processing systems. In order to improve the quality of these devices, it is necessary to develop non-linear optical materials which have a high non-linear susceptibility (hereinafter referred to as ".chi..sup.(3) ") and high speed responsibility.
Inorganic materials have conventionally been employed, but organic materials having E electron conjugated systems have attracted much attention recently because of their high responsibility and high non-linear susceptibility due to the presence of .pi. electrons. There have been known, for instance, polymer systems such as polyacetylene and polydiacetylene; and low molecular systems such as azomethines. All of these compounds have one-dimensional .pi. electron conjugation and the maximum .chi..sup.(3) thereof is 10.sup.-8 esu due to the response of these electron systems. However, this value is substantially determined on the basis of the presence or absence of the one-dimensional .pi. electron conjugation and is not dependent upon the chemical structure of a specific compound selected. Thus, there has not yet been discovered any organic materials having .chi..sup.(3) substantially greater than 10.sup.-8 esu. Moreover, the quality indices of non-linear optical devices are given by .chi..sup.(3) /(.alpha..tau.) (wherein .tau. is a response speed and .alpha. is the coefficient of light absorption) and, therefore, the indices must be increased as high as possible. However, there is a limit in the quality of these organic materials having one-dimensional .pi. electron conjugation. For this reason, it has been required for the discovery of other materials different from the foregoing materials.